The present invention relates to an ultrasonic Doppler blood-flow meter used in the medical field and capable of simultaneously displaying data of a desired blood-stream in a living body and a B-mode diagnostic image in real time.
In recent years, the ultrasonic Doppler blood-flow meter has found its widespread use in the medical field including diagnoses of the cardiology and the vascular organ. The ultrasonic Doppler blood-flow meter utilizes a phenomenon that an ultrasonic pulse signal transmitted into a living body undergoes a frequency deviation due to the Doppler effect occurring when the pulse signal is reflected by a moving object such as a blood stream and it is so constructed as to measure a speed of the blood stream acting as a reflector by detecting a Doppler deviated frequency and to permit easy observation, from the body surface, of a blood stream speed distribution in the living body by displaying a result of the measurement. Conventional ultrasonic Doppler blood-flow meters will now be described with reference to the accompanying drawings.
FIG. 1 is a functional block diagram showing a conventional ultrasonic Doppler blood-flow meter. Referring to FIG. 1, a probe 1 converts a pulse signal into an ultrasonic pulse signal, transmits the ultrasonic pulse signal into a living body 14 and converts an ultrasonic wave reflected and received from the inside of the living body into an electrical signal. A drive circuit 2 transmits the pulse signal to the probe 1 to drive it. A transmission timing circuit 3 generates a timing signal for the drive circuit 2 to generate pulses. A receiving circuit 4 amplifies an echo signal received from the probe 1. A phase detector 5 performs phase detection of the echo signal delivered from the receiving circuit by using reference signals. A reference signal generation circuit 6 generates a reference signal to which frequency and phase of the reference signals used for the phase detection in the phase detector 5 are referenced. A gate signal generation circuit 7 generates a gate signal during an interval of time corresponding to a propagation time required for the ultrasonic wave to propagate between the transmitting/receiving surface of the probe 1 and a portion to be examined Analog switches 8a and 8b enable phase-detected signals from the phase detector 5 to be passed during only an interval or duration of the gate signal generated by the gate signal generation circuit 7. Integrators 9a and 9b integrate the phase-detected signals having passed through the analog switches 8a and 8b to determine the summation of the phase-detected signals Accordingly, by repeating transmission/reception of the ultrasonic pulse signal, a Doppler deviated signal can be obtained. Sample-and-hold circuits 10a and 10b hold a result of an integral operation until a result of the next integral operation is obtained, in order to permit resetting to be done before the integrators 9a and 9b perform integral operations. High-pass filters 11a and 11b remove a signal component of less than several of tens of Hz or several of hundreds of Hz, that is, a clutter component from the Doppler deviated signals produced by the integrators 9a and 9b. A frequency analyzer 12 analyzes the frequency of the Doppler deviated signals passed through the high-pass filters 1a and 11b. A display unit 13 displays results of frequency analysis.
The above construction will now be described in greater detail by referring to the operation thereof.
An electrical pulse signal generated by the drive circuit 2 using a timing signal generated from the transmission timing circuit 3 as a trigger is converted by the probe 1 into an ultrasonic pulse signal which in turn is transmitted into the living body 14 acting as an object to be examined. The ultrasonic pulse signal propagates within the living body and it is reflected at a portion at which the acoustic impedance changes to reach the probe 1 and converted into an electric signal. The thus obtained echo signal is amplified by the receiving circuit 4 to a suitable extent and then applied to the phase detector 5 so as to undergo phase detection. Reference signals for phase detection generated from the reference signal generation circuit 6 are two signals Vx and Vy being in synchronism with a timing signal of the transmission timing circuit 3 and degrees dephased mutually. These signals are 90 indicated by the following equations (1) and (2): EQU Vx=coswt . . . . . (1) EQU Vy=sinwt . . . . (2)
The echo signal, E, is indicated by the following equation EQU E=Acosw(1+.alpha.)t . . . . (3)
where A is echo intensity and .alpha. is frequency shift coefficient. In the phase detector 5, E in equation (3) is multiplied with each of the signals Vx and Vy in equations (1) and (2) and there result the following equations (4) and (5): ##EQU1##
In the right side of equations (4) and (5), the first term is of a low frequency of about several of kHz or less and the second term is of a high frequency of several of MHz. Accordingly, when the analog switches 8a and 8b are turned on during only a sampling volume obtained from these signals and signals confined within this interval are integrated by the integrators 9a and 9b, the second term are extinguished and a value of the first term which is proportional to a deviation at an instant can be obtained. The data is held by the sample-and-hold circuits 10a and 10b so that a stepped signal representative of a discrete-time Doppler deviated signal is obtained.
The thus obtained Doppler deviated signal contains blood flow data and a component called a clutter as well which is due to an echo from a tissue of living body such as a vascular wall, the clutter component being as large as about 40 dB of the blood flow component. Therefore, for the sake of expanding the dynamic range of the frequency analyzer 12, elimination of the influence due to an echo from a living body tissue is of significance. The frequency of Doppler deviated signal of the echo from living body tissue is in general several of tens of Hz or less and is lower than that of the blood flow component. Therefore, by removing low frequencies by means of the high-pass filters 11a and 11b, the influence of the echo from living body tissue can be eliminated. The thus obtained Doppler deviated signal is subjected to frequency conversion by means of the frequency analyzer 12 and then displayed on the display unit 13.
From the standpoint of the dynamic range of the integrators 9a and 9b, however, the integrators 9a and 9b are applied with the coexistence of a strong Doppler deviated signal due to the echo from living body tissue and a weak Doppler deviated signal due to the blood flow and therefore, when the weak Doppler deviated signal due to the blood flow is amplified at a large gain, the integrators 9a and 9b are inconveniently saturated by the Doppler deviated signal stemming from the living body tissue. When saturated, the Doppler deviated signal of blood flow at that portion are extinguished and in addition, the waveform is distorted to generate unwanted frequency components. To cope with such a problem, a construction as described in, for example, JP-A-61-265131 and JP-A-62-155836 has been proposed. This prior art is illustrated in a functional block diagram of FIG. 2 of the accompanying drawings and as shown, it structurally differs from the conventional example shown in FIG. 1 by the provision of DC feedback circuits 15a and 15b for feeding back the output signals of the sample-and-hold circuits 10a and 10b to the inputs of the analog switches 8a and 8b.
FIG. 3 shows an example of a specific circuit arrangement of the analog switch 8a or 8b, integrator 9a or 9b, DC feed back circuit 15a or 15b and sample-and-hold circuit 10a or 10b. As is clear from FIG. 2, two channels of this circuit arrangement are needed. The integrator 9a or 9b includes a resistor (R1) 101, a capacitor (Co) 102, an operational amplifier (OP1) 103 and an analog switch 104 and the DC feedback circuit 15a or 15b includes a resistor (R2) 105, a resistor (Rf) 106, a capacitor (Cf) 107, an operational amplifier (OP2) 108 and an analog switch 109. Denoted by 110 is a feedback gain control circuit. The sample-and-hold circuit 10a or 10b has the function to amplify at a gain of -A times.
With the above construction, the operation will now be described with reference to a timing chart of FIG. 4.
A signal subjected to phase detection by means of the phase detector 5 in a similar manner to that in the foregoing conventional example is passed through the analog switch 8a or 8b which is turned on during a gate interval t1-t2 and stored in the capacitor 102 of the integrator 9a or 9b. Since the analog switch 104 is turned on by a RESET signal in advance of the gate interval, a value resulting from integral during only the gate interval is obtained. When the gate interval ends, the integrated value of the integrator 9a or 9b is held in the sample-and-hold circuit 10a or 10b and at the same time -A times amplified thereby and then applied to the DC feedback circuit 15a or 15b through the analog switch 109. On-time tf of the analog switch 09 is determined in accordance with a cut-off frequency fc of the entire circuitry of FIG. 3.
The DC feedback circuit 15a or 15b is an integrator, namely, a kind of low-pass filter. The input signal to the DC feedback, circuit 15a or 15b is inverted in phase by being -A times amplified by means of the sample-and-hold circuit 10a or 10b and therefore the output signal from the DC feedback circuit 15a or 15b corresponds to a phase inversion of a DC component and an extremely low-frequency component of the output signal of the integrator 9a or 9b. Since the output signal of the DC feedback circuit 15a or 15b is fed back to the input of the analog switch 8a or 8b, the fed back signal is inputted along with the phase detected signal to the integrator 9a or 9b when the analog switch 8a or 8b is turned on, to cancel out the DC component and extremely low-frequency component contained in the Doppler deviated signal. Through this operation, the entire circuitry of FIG. 3 acts as a high-pass filter having a cut-off frequency fc given by the following equation (6): ##EQU2## where t.sub.g =t.sub.2 -t.sub.1.
As described above, by operating the circuit arrangement of FIG. 3, saturation of the integrators 9a and 9b due to the echo from living body tissue can be mitigated. In the foregoing example, the output signal of the sample-and-hold circuit 10a or 10b is applied to the DC feedback circuit 15a or 15b but similar results can be obtained by applying the output signal of the integrator 9a or 9b to the DC feedback circuit 15a or 15b.
Now, the principle of an ultrasonic Doppler blood-flow meter for simultaneously displaying both of a B-mode image and a Doppler spectrum in real time, which meter will hereinafter be referred to as a simultaneous Doppler type meter, will be described.
In the simultaneous Doppler type, sequence of switching between the B mode and the Doppler mode can be conceived in various ways but a scheme in which the sequence of the B mode and Doppler mode is alternately switched at each TX pulse, hereinafter called an alternate scheme, is generally employed. In the alternate scheme, however, the sampling interval of the Doppler deviated signal is doubled, raising a problem that the maximum blood flow speed measurable without aliasing is halved.
A different scheme from the alternate scheme has been contrived in which switching between the B mode and Doppler mode is effected at intervals of several of tens or several of hundreds of TX pulses. This latter scheme will hereinafter be called a chopper scheme. In the chopper scheme, the sampling interval remains unchanged but the Doppler spectrum is interrupted during the B-mode period and some compensation is needed.
In a conventional serial Doppler type based on the chopper scheme, the deficit of signal due to the B-mode sequence intervening between the preceding Doppler sequence and the succeeding Doppler sequence leads to the following problems.
FIG. 5 shows an example of output waveform of the sample-and-hold circuit in the simultaneous Doppler type based on the chopper scheme. The sample-and-hold circuit delivers an output signal as shown at E0 when the DC feedback circuit is not provided. This waveform contains a small-amplitude Doppler deviated signal of blood stream superposed on a large-amplitude Doppler deviated signal of living body tissue but its value is zero during B-mode period because of the absence of any input signal. As soon as the B mode switches to the Doppler mode, a Doppler signal develops, causing a large jump of signal at an instant of switching. In an output signal Eo of the sample-and-hold circuit delivered out thereof when the DC feedback circuit is provided, unwanted frequency components due to the jump of signal are generated.
Further, in many applications of the ultrasonic Doppler blood-flow meter, control of changing the gate position, gate width, amplitude of transmission pulse and mu-factor of receiving amplifier is carried out by placing the ultrasonic Doppler blood-flow meter in operated condition while monitoring the status of an object to be examined in living body. When the changing control is effected, however, discontinuity takes place between data before change and data after change. Specifically, when the gate position or gate width is changed, the position or magnitude of sample volume changes and when the transmission pulse output signal or mu-factor of receiving amplifier is changed, the amplitude of signal changes. In the presence of the thus occurring discontinuity of signal, unwanted frequency components are generated owing to the jump of signal taking place at the discontinuous plane.
In addition to the above, the Doppler signal abruptly develops when a freeze of the operation of the apparatus is released, and the resulting jump of signal gives rise to occurrence of unwanted frequency components.